Measurement of the properties of a gas with a view to determining the composition generally resorts to associating a chromatography column, which allows separation in time of different species contained in the gas to be analyzed according to their affinity with a stationary phase laid out in the column, and one or several detectors placed at the outlet of said column, each detector being adapted so as to measure a particular property of the species of the gas to be analyzed.
At the present time, the detectors used are intended for measuring physical properties of the gas.
Among the detectors frequently used, mention may be made of Flame Ionization Detectors (FID) which allow measurement of the combustion energy of a gas; Thermal Conductivity Detectors (TCD) which allow measurement of the heat conductivity of a gas; chemical detectors known under the name of «chemiresistors» which allow evaluation of the chemical affinity of the gas towards a chemical species.
Gravimetric detectors, including resonators of the MEMS (electromechanical microsystem) or NEMS (electromechanical nanosystem) type also allow evaluation of the chemical affinity of the gas towards a functionalized layer deposited at the surface of the resonator [Fanget 2011].
Indeed, the adsorption of the species contained in the gas on the functionalized layer causes change in the mass and therefore in the resonance frequency of the resonator, this change in the resonance frequency allowing determination of a physical property of the gas.
NEMS detectors are particularly promising as detectors coupled with a chromatography column [Arcamome 2011].
However, depending on the gas to be analyzed, the gravimetric measurement carried out by means of these resonators has certain limitations.
On the one hand, NEMS detectors do not allow detection of volatile species since these species are not adsorbed on the functionalized layer of the resonators.
Moreover, NEMS detectors do not allow detection by gravimetry of rare or inert gases (such as N2, O2, Ar, He, etc.), since there is no possible chemical interaction with a functionalized layer deposited on the resonator.
On the other hand, for highly concentrated gases, saturation of the functionalized layer by the concentrated species may occur especially when the functionalization layers are thin (from a few nanometers to a few tens of namometers), which does not allow a reliable measurement.
The saturated concentrations in the chemical functionalization layers depend on the type of layer, on the layer thickness and especially on the type of gas involved. They may for example be of several tens of %.
In other words, these NEMS detectors are mainly adapted for detecting heavy species and/or moderately concentrated species.
A use of MEMS or NEMS detectors for measuring fluidic interactions between the resonator and a gas which are expressed by damping of the vibrations of the resonator according to the molar mass of the gas has been described in [Xu 2006].
Thus, the measurement of the change in the resonance frequency of the resonator allows determination of the molar mass of the gas, even in the absence of chemical interaction between the resonator and the gas.
However, such a measurement can only be carried out for sufficiently concentrated gases.
Now, in the design of a gas analysis system, it is generally sought to make this system «universal», i.e. capable of analyzing any gas, notably complex mixtures.